Temperature dependence of the vibrational relaxation processes in natural and isotopically pure 32S8: Effect of the isotopic impurities on infrared phonon lifetimes

Abstract

We have measured the temperature dependence of the infrared bandwidth of several lattice phonons and internal vibrons in natural and 32S isotopically pure orthorhombic sulfur crystals, using a high resolution FTIR interferometer. The experimental data were analyzed in terms of anharmonic phonon–phonon coupling processes and of contributions to the bandwidth due to the presence of randomly distributed isotopic impurities. The same three-phonon processes that contribute to the lifetime of the Raman bands were found to be active also for the infrared ones in the isotopically pure crystal. In parallel to the increase of the two-phonon density of states, the third-order anharmonic coupling coefficients for phonon decay processes were found to increase from the lowest to the highest lattice mode. The effect of the isotopic impurities was explained in terms of two contributions, the first, which is temperature independent, due to pure harmonic scattering processes and the second due to the combined effect of anharmonicity and scattering processes, which is instead dependent on the temperature. The contribution due to the presence of impurities was found to dominate the width of the lattice modes in the natural crystal at low temperatures. This purely harmonic contribution can be in some cases almost one order of magnitude larger than that due to the normal anharmonic decay processes in the isotopically pure crystal. Only the width of the factor group component of infrared and Raman vibrons close to the frequency of the isolated molecule was found to be affected by the presence of impurities through a resonant scattering mechanism.

The role of anharmonic phonons in under-barrier spin relaxation of single molecule magnets

“…with . Such temperature dependence of is in qualitative agreement with both the experimental and theoretical homogeneous linewidth temperature behaviour 15 17 19 . This choice of Δ corresponds to the case where the phonons rapidly become over damped as T increases and it can be considered as opposite to the harmonic limit.…”

“…The inset of Fig. 1c schematically illustrates the typical temperature dependence of Δ α , which takes a small constant value for T < T * and then becomes linear above the threshold temperature, T * (refs 15 , 16 , 17 ). The spin dynamics then qualitatively depends on the value of U 0 with respect to k B T * (see Fig.…”

How do phonons relax molecular spins?

“…We simulate this effect by assuming the linear relation ℏΔ α q = γ T between the line width and the temperature. This model is justified by perturbation theory arguments ( 25 ). Figure 3 shows that this model has the effect of increasing the τ versus T power law of one unit and, depending on the size of the coefficient γ, speeding up the overall spin relaxation rate.…”

Mode Assignment of Sulfur α-S8 by Polarized Raman and FTIR Studies at Low Temperatures

“…Vibrational spectroscopic data were collected by Raman scattering from natural sulfur (S nat ) crystals or powder at 300, 2,5,6 100, 3 77, 11 40, 14 30, 7 and 16 K 15 and by IR absorption on S nat crystals at 100, 4 30, 7 10, 16,17 and 4 K. 10 In addition, polarized spectra from oriented single crystals were measured both by Raman 5,6,7,15 and IR techniques. 7,16,17 Isotope-free R-32 S 8oriented single crystals were investigated only recently by polarized Raman 15 and polarized FTIR techniques.…”